Nitrocellulose, which is processed into various types of propellants, is manufactured from cellulose feed stocks consisting of fine fibers of cotton linters or macerated wood pulp.
The present method used to manufacture nitrocellulose involves the nitration of cellulose with a mixture of nitric acid, sulfuric acid and water. This process requires a purification operation that includes neutralization and stabilization steps which involve long boiling and washing operations, consuming large quantities of water, steam and electricity. Although the resulting product is satisfactory, the process is long and costly and large quantities of contaminated water are produced, causing pollution problems. The method of purification, however, does meet the pollution abatement requirements, but at a high cost.
Research has been conducted in an effort to improve the total process and the purification operation particularly so that they are more efficient and meet the pollution abatement requirements.
Among the difficulties in purifying the nitrocellulose produced by the nitric acid- sulfuric acid nitration of cellulose are the sulfate esters formed during the nitration and the presence of sulfuric acid trapped in the nitrocellulose fibers.
One method used in an attempt to overcome the problem is pressure digestion system. However, this method is unsatisfactory since the resulting product had a low viscosity and thus does not have the proper properties to enable it to be used in a large number of propellants.
Efforts have been made also to develop a process which eliminates the need for using sulfuric acid. It was found that nitric acid without sulfuric acid satisfactorily nitrated the cellulose, but the resulting nitrocellulose was not within the specifications required for use in propellants. However, it was found that if cellulose was nitrated with nitric acid alone in a vapor phase reaction, the problems of hornification and gelatinization are avoided and the nitrocellulose obtained is of acceptable quality. This product is also very easy to neutralize and stabilize when compared to the product obtained by the mixed acids method.
The advantages of the vapor or gas phase reaction because of the absence of sulfuric acid are:
(a) Easy recovery of nitric acid and significant reduction in the volume of nitrating acid required; PA1 (b) Elimination of the expensive and complicated system for the separation, recovery and concentration of the mixed acids; PA1 and (c) Simplification of the purification of nitrocellulose by shortening the boiling time and reducing energy, steam and water requirements. PA1 (a) The nitration step can be made continuous rather than batch; PA1 (b) The purification step can be made continuous using pressure digestion. This is possible because of the much shorter boiling time that is required to obtain a stable material. Under these conditions, the viscosity reduction of the product below acceptable standards is minimal; and PA1 (c) The quantity of acid handled in the vapor or gas phase process is much less than in the mixed acid process with the result that the process and equipment required are significantly simplified.
In addition, the vapor or gas phase nitration process is advantageous in other ways, for example:
The gas phase process for the nitration of cellulose, despite its advantages, has heretofore not been totally satisfactory since the nitrocellulose produced lacked homogeniety as manifested by erratic solubilities which affected propellant mixing. This was caused by a failure of the nitrating acid to reach all reaction sites of the cellulose fibers. The fact that the cellulose fibers can be properly nitrated by a gas phase reaction is demonstrated by a homogeneous product that can be made by a batch process vapor phase nitration of cellulose.
The above demonstrates that there is a need for a reactor which will permit a continuous gas or vapor phase reaction of nitric acid and cellulose to produce a nitrocellulose which is homogeneous and suitable for use in propellants.